Waking up and seeing a green pool is a sight no pool owner wants. But why do they turn green like that and how do I get rid of it? The reason pools can turn green is because of algae build up and the only sure way to kill algae and prevent a green pool is chlorine. You have to make sure you are using enough chlorine though. Many times we hear, “But I put four bags of shock in the pool, and it’s still green” Well, four bags of shock should kill algae in a pool under 20,000 gallons, If they are added at once, not a bag at a time. The key to getting rid of algae, is “super chlorination.”

You must add enough chlorine to attack the “flower” of the algae. Usually twice the normal dosage. If there is algae on the walls and floor, that algae must be brushed as soon as the pool is shocked. Brushing will open the pores of the bloom and allow the chlorine to penetrate and kill the organism. After the pool has been shocked and brushed, let the dead algae settle for a day, then vacuum the pool to waste. Get rid of it, and keep your chlorine residual between 1ppm and 3ppm, to keep it gone. Wiki answers is also very helpful